1. Technical Field
The present disclosure relates to a magnetic-field sensor, in particular one comprising anisotropic magnetoresistive elements.
2. Description of the Related Art
Magnetic-field sensors, in particular anisotropic-magnetoresistive (AMR) sensors, are used in a plurality of applications and systems, for example in compasses, in systems for detecting ferromagnetic materials, in the detection of currents, and in a wide range of other applications, thanks to their capacity for detecting natural magnetic fields (for example, the Earth's magnetic field) and magnetic fields generated by electrical components (such as electrical or electronic devices and lines traversed by electric current).
In a known way, the phenomenon of anisotropic magnetoresistivity occurs within particular ferromagnetic materials, which, when subjected to an external magnetic field, undergo a variation of resistivity as a function of the characteristics of the magnetic field applied. Usually, said materials are shaped in thin strips so as to form resistive elements, and the resistive elements thus formed are electrically connected together to form a bridge structure (typically a Wheatstone bridge).
It is moreover known to produce AMR magnetic sensors with standard semiconductor-micromachining techniques, as described for example in the document No. U.S. Pat. No. 4,847,584. In particular, each magnetoresistive element can be formed by a film of magnetoresistive material, such as for example permalloy (a ferromagnetic alloy containing iron and nickel), deposited to form a thin strip on a substrate made of semiconductor material, for example silicon.
When an electric current is made to flow through a magnetoresistive element, the angle θ between the direction of magnetization of said magnetoresistive element and the direction of the flow of the current affects the effective resistivity value of the magnetoresistive element itself so that, as the value of the angle θ varies, the value of electrical resistance varies (in detail, said variation follows a law of the cos2 θ type).
For example, a direction of magnetization parallel to the direction of the flow of current results in a maximum value of resistance to the passage of current through the magnetoresistive element, whereas a direction of magnetization orthogonal to the direction of the flow of current results at a minimum value of resistance to the passage of current through the magnetoresistive element.
AMR magnetic sensors further include a plurality of straps integrated in the sensors themselves (typically two straps), the so-called “set/reset strap” and “offset strap”, which are designed to generate, when traversed by a current of an appropriate value, a magnetic field that is coupled in a direction perpendicular to the direction of detection of the sensors and in the direction of detection of the sensors, respectively; in this regard, see for example the document No. U.S. Pat. No. 5,247,278.
The set/reset strap has the function of varying, alternating it, the sense of magnetization of the magnetoresistive elements in a first pre-defined direction (the so-called “easy axis” or EA). In use, the variation of the sense of magnetization is obtained by applying to the magnetoresistive element, via the set/reset strap, a magnetic field of an appropriate value for a short period of time such as to force arbitrarily the orientation of the magnetic dipoles of the magnetoresistive element in the first pre-defined direction (“set” operation), and then by applying to the magnetoresistive element a second magnetic field, similar to the previous but with opposite sense, so as to force the orientation of the magnetic dipoles of the magnetoresistive element once again in the first pre-defined direction, but with opposite sense (“reset” operation). The set and reset operations have the function of sending each magnetoresistive element of the AMR sensor into a respective single-domain state before operating the AMR sensor, for example in order to carry out operations of sensing of an external magnetic field. The set and reset operations are used because only in the single-domain state are the fundamental properties of linearity, sensitivity, and stability of the magnetoresistive elements controlled and repeatable. The aforementioned set and reset operations are known and described in detail, for example, in the document No. U.S. Pat. No. 5,247,278.
The offset strap is normally used for operations of compensation of offsets present in the AMR sensor (on account of mismatch in the values of the corresponding electrical components), self-test operations, and/or operations of calibration of the AMR sensor. In particular, the value of the electrical quantities at output from the AMR sensor is, in the presence of the offset strap, a function both of the external magnetic field to be detected and of the magnetic field generated as a result of a current circulating in the offset strap. The offset strap is formed by turns of conductive material, for example metal, arranged on the same substrate as that on which the magnetoresistive elements of the sensor and the set/reset strap are provided (in different metal layers), and is electrically insulated from, and set in the proximity of, said magnetoresistive elements. The magnetic field generated by the offset strap is such as to force partially the orientation of the magnetic dipoles of each magnetoresistive element in a second pre-defined direction (the so-called “hard axis” or HA), orthogonal to the first pre-defined direction.
FIG. 1 shows, in top plan view, a layout provided by way of example of an integrated magnetic-field sensor 1 of a known type comprising a plurality of magnetoresistive elements, connected to one another so as to form a Wheatstone bridge, for example as described in the document Nos. U.S. Pat. No. 5,247,278 and U.S. Pat. No. 5,952,825, and manufactured according, for example, to what is described in the document No. U.S. Pat. No. 4,847,584.
More in particular, each magnetoresistive element has a structure of the barber-pole type. The barber-pole structure for magnetoresistive elements is known. In this case, each magnetoresistive element is formed by a plurality of magnetoresistive sub-elements arranged substantially in line with one another and connected to one another by means of connection elements with high electrical conductivity (for example, ones made of aluminum, copper, silver, or gold). The connection elements are arranged adjacent to, in direct electrical contact with, each magnetoresistive sub-element and inclined by a certain angle α (typically, α=45°) with respect to the axis of spontaneous magnetization of the magnetoresistive element.
The magnetic-field sensor 1 is formed on a semiconductor substrate 2 by means of a technological process of a known type. Four magnetoresistive elements 4, 6, 8, and 10, in the form of strips made of ferromagnetic material (for example, deposited thin film comprising an Ni/Fe alloy), in barber-pole configuration, are arranged to form a Wheatstone bridge. For each magnetoresistive element 4, 6, 8, 10, the magnetoresistive strips that form it are connected together in series. With reference to FIG. 1, the magnetoresistive elements 4, 6, 8, 10 are interconnected and connected to pads 21, 22, 23, 24, and 25. The pad 21 is connected to the magnetoresistive element 4 by means of a conductive path 11, and the magnetoresistive element 4 is connected to the magnetoresistive element 6 by means of a conductive portion 16. The conductive portion 16 is electrically connected to the pad 22 by means of a respective conductive path 12. The magnetoresistive element 6 is then connected to the magnetoresistive element 10 by means of a conductive portion 18, and the conductive portion 18 is electrically connected to the pad 23 by means of a respective conductive path 13. The magnetoresistive element 10 is interconnected to the magnetoresistive element 8 by means of a conductive portion 17, and the conductive portion 17 is electrically connected to the pad 24 by means of a respective conductive path 14. The pad 25 is connected to the magnetoresistive element 8 by means of a conductive portion 15.
A resistive Wheatstone-bridge structure is thus formed, which provides a magnetic-field sensor 1 sensitive to components of magnetic field having a direction perpendicular to the strips made of ferromagnetic material that form the magnetoresistive elements 4, 6, 8, 10. The pad 21 is connected to the pad 25, to form a common pad so as to connect the magnetoresistive element 4 and the magnetoresistive element 8 electrically together.
In use, an input voltage Vin is applied between the pad 22 and the pad 24. Reading of the output voltage Vout is carried out between the pad 21 (common to the pad 25) and the pad 23.
With reference to FIG. 1, the magnetic-field sensor 1 further comprises a first strip of electrically conductive material, which extends on the substrate 2 and is insulated from the latter by means of a layer of dielectric material (not shown in detail in the figure). Said first strip of electrically conductive material forms a first winding 19, of a planar type, which extends in a plane parallel to the plane in which the magnetoresistive elements 4, 6, 8, 10 lie and is electrically insulated from the magnetoresistive elements 4, 6, 8, 10.
The magnetic-field sensor 1 further comprises a second strip of electrically conductive material, which extends on the substrate 2, and is insulated from the latter and from the first winding 19 by means of a layer of dielectric material (not shown in detail in the figure). Said second strip of electrically conductive material forms a second winding 20, of a planar type, which extends between a terminal 20a and a terminal 20b in a plane parallel to the plane in which the magnetoresistive elements 4, 6, 8, 10 and the first winding 19 lie and is electrically insulated from the magnetoresistive elements 4, 6, 8, 10 and from the first winding 19.
The first winding 19 is used when it is desired to generate a magnetic field of known intensity interacting with the magnetic-field sensor 1, with purposes of biasing, calibration, and/or compensation of possible offsets due to the presence of undesirable external magnetic fields. In the latter case, the effect of the magnetic field generated by the first winding 19 on the output signal Vout of the magnetic-field sensor 1 is that of balancing the output signal due exclusively to the undesirable external field in order to generate a zero output signal.
In use, when the first winding 19 is traversed by electric current, a magnetic field is generated the lines of force of which have a direction parallel to the plane in which the magnetoresistive elements 4, 6, 8, 10 lie, or in any case a direction parallel to the direction of sensitivity of the magnetoresistive elements 4, 6, 8, 10.
On account of the variability of the process of manufacture of the magnetoresistive elements 4, 6, 8, 10, said magnetoresistive elements 4, 6, 8, 10 can have structural characteristics different from one another. This generates an offset signal Voff, superimposed on the useful output signal, intrinsic to the magnetic-field sensor 1, which causes a reduction of the sensitivity of the magnetic-field sensor 1 during use. Said offset signal Voff can be eliminated by appropriately operating the second winding 20. In greater detail, during use, current pulses are made to flow in the second winding 20 with a direction opposite to one another (by appropriately biasing the terminals 20a and 20b of the second winding) so as to generate respective magnetic fields defined by respective field lines having senses opposite to one another. Said magnetic fields have an intensity such as to re-orient the magnetic dipoles of the magnetoresistive elements 4, 6, 8, 10 according to the field lines generated, in particular with a sense defined by the sense of the lines of the magnetic field generated.
Following upon a first current pulse (referred to as “set pulse”) through the second winding 20, a first magnetic field Hsi is generated such as to orient the magnetic dipoles of the magnetoresistive elements 4, 6, 8, 10 according to a first sense.
Following upon a second current pulse (referred to as “reset pulse”) through the second winding 20, a second magnetic field HS2 (of intensity, for example, equal to that of the first magnetic field HS1) is generated such as to orient the magnetic dipoles of the magnetoresistive elements 4, 6, 8, 10 in a second sense.
The AMR sensor described with reference to FIG. 1 hence requires, in order to be correctly operated, at least two straps (the set/reset strap and the offset strap).
The presence of said straps complicates the process of fabrication of the AMR sensor, and the manufacturing cost thereof increases in so far as it uses masks dedicated to formation of two different straps on different metal levels and to formation of metal connections (via holes) through the wafer for providing the electrical supply for biasing said straps.